A fluoroalkyl iodide (hereinafter sometimes referred to as RfI) is useful as a material for synthesis of a fluoroalkyl acrylate constituting a water/oil repellent latex, a material for synthesis of a fluorinated surfactant, etc.
Recently, EPA (US Environmental Protection Agency) has pointed out that a compound having a perfluoroalkyl group having at least 8 carbon atoms (hereinafter, the perfluoroalkyl group may be presented also as a RF group) is likely to be decomposed in vivo or in the environment, whereupon the decomposition product is likely to be accumulated, i.e. the environmental load tends to be high. Therefore, it is required to reduce the compound having a Rf group having at least 8 carbon atoms as far as possible.
In the fluoroalkyl iodide, the carbon chain length of Rf desired to obtain water/oil repellency is usually C4 or longer, although it may depend also upon the purpose of use. For production of RFI having such a carbon chain length, chain length elongation by addition of a taxogen employing a short chain RfI as a telogen i.e. telomerization is utilized. As the taxogen, usually tetrafluoroethylene CF2CF2 (hereinafter sometimes referred to as TFE) is used, and thus a telomer fluoroalkyl iodide RFI is obtained as Rf(CF2CF2)nI (n is the degree of polymerization). The starting material telogen RfI is typically C2F5I, which is synthesized from tetrafluoroethylene, IF5 and I2.
By simply carrying out the telomerization, a 1:1 addition product of a telogen and a taxogen mainly forms, and only a very small amount of a telomer having a chain length more highly elongated will be obtained. It has been known that the efficiency of formation of C6-12 RFI will be increased by use of a free radical-generating catalyst such as a peroxide for the telomerization (for example, Patent Document 1). The publication also discloses use of a telogen mixture of C2F5I and C4F9I. The reaction in this publication is a liquid phase reaction conducted in one step.
In addition to the reaction by means of free radical generation, telomerization by a catalytic reaction utilizing the redox system and telomerization by thermal reaction have been known. In order to reduce the compound having at least 8 carbon atoms, formation of a telomer having a narrow distribution in a specific chain length range or a telomer having a single chain length is required to obtain desired characteristics. However, in each of the above methods, selectivity for a single chain length tends to be low, control of the chain length is difficult, and a telomer mixture having a broad chain length distribution will be obtained.
It has been known that in the thermal reaction of reacting a telogen and a taxogen in a vapor phase, the proportion of formation of a long chain telomer exceeding C14 can be reduced by increasing the ratio of telogen/taxogen. In order to reduce the proportion of formation of a long chain telomer in the vapor phase reaction, a continuous process of dividedly supplying the taxogen TFE from an inlet of a tubular reactor and from another portion, has been proposed (for example, Patent Document 2). This publication also discloses that the chain length distribution can be narrowed when telomers having chain lengths of C4 and C6 which are sequentially formed are used as the telogen together with the C2 starting material, as compared with a case where the C2 starting material is used alone as the telogen.
Further, it has been proposed that in the above thermal telomerization in the vapor phase, a telomer having a chain length shorter than the final chain length is recycled to a predetermined zone of the reactor so as to improve the selectivity for the carbon chain length (for example, Patent Document 3).
According to the above vapor phase telomerization, a fluoroalkyl iodide telomer having a relatively narrow chain length distribution can be obtained, but the telomerization has to be carried out under extremely restricted conditions with regard to e.g. introduction of the material taxogen or the recycled telomer to the reactor. Further, due to the vapor phase reaction, there is such a problem that a perfluoroalkyl compound is formed as an impurity by coupling of fluoroalkyl radicals to be generated during the reaction.
On the other hand, a liquid phase reaction employing a catalyst is advantageous in view of energy since the reaction temperature is inherently low as compared with the above vapor phase reaction, and is advantageous in that a thermally unstable taxogen (TFE) is less likely to be decomposed. A process is disclosed wherein the liquid phase reaction is carried out in a slender cylindrical reaction space, the reaction mixture which left the cylindrical reaction space is separated, and a telomer having the reaction progressed is drawn, and on the other hand, a telomer having no desired chain length and an unreacted material are recycled to the initial stage of the reaction system (Patent Document 4). Patent Document 4 discloses that the proportion of formation of C8 and longer telomers can be improved. Further, the waste gas can be reduced by the recycle.
Further, a process by a liquid phase reaction has been proposed which comprises separating the reaction mixture of the initial material telogen and TFE into three fractions, and subjecting a second fraction having a degree of polymerization of TFE lower by 1 than the desired degree of polymerization to reaction in a second reactor (Patent Document 5). In this process, a telomer mixture having at least a desired degree of polymerization is obtained by a two-step reaction. No special reactor is required for each of the two reactors, and the reactor may be an autoclave or the like.
The above telomerization by means of a liquid phase method is advantageous over a vapor phase method in view of operation and energy, and in that no impurities such as a perfluoroalkyl compound will be formed as by-products. By the telomerization by means of a liquid phase method, a telomer having a desired chain length or longer will be obtained, but it tends to be difficult to control the chain length, and particularly it tends to be difficult to suppress formation of a telomer having a chain longer than the desired chain length. Even when a known reaction method is applied so as to control the chain length, a telomer having a broad chain length distribution will be obtained in fact. Further, in continuous operation, a telomer having the chain length controlled will be obtained by removing an unreacted product from the reaction product as far as possible, but such remarkably decreases the reaction efficiency. As mentioned above, it is difficult to obtain a fluoroalkyl iodide telomer having a narrow chain length distribution in a desired range, particularly a fluoroalkyl iodide telomer having an aimed single carbon chain length with high production efficiency, by a telomerization process by means of a liquid phase method.